Specifying it in web.xml should be enough unless there is that default propery file available as web.xml value is set only if there is no other callback registered already. So I can imagine situation that callback manager found the property file first and registered default one and web.xml value is skipped. So please ensure that the default property file is not there. If you find it somewhere in the packages shipped with jBPM please file a jira issue for that to be cleaned up.
Thanks